Structure, Resolution, and Racemization of Decakis(dichloromethyl)biphenyl

Decalds(dichloromethyl)biphenyl (1) was obtained by photochlorination of decamethylbiphenyl. Crystals of the 1:1 THF solvate are monoclinic, space group P2₁/n, a = 17.296 (5) Å, b = 10.091 (3) Å, c = 21.731 (5) Å, 8 = 96.79 (2)°, Z = 4. The disposition of the dichloromethyl groups on both rings is all-geared, and the molecule has approximate C₂ symmetry. The ¹H NMR solution spectrum of 1 is consistent with a time-averaged C₂ conformation. The molecule assumes a curved shape that is well reproduced by empirical force field calculations. Homo- and heterodirectional relationships between subcycles of the molecular model are discussed. Enantiomers of 1 were separated by HPLC on a column of cellulose tris(3,5-dimethylphenylcarbamate). The biphenyl racemizes with a barrier (ΔG) of 23.7 kcal mol⁻¹; the threshold mechanism for the enantiomerization most likely involves internal rotation of the dichloromethyl groups rather than rotation of the pentakis-(dichloromethyl)phenyl groups about the central biphenyl bond.